1. Field of the Invention
The present invention relates to a pulse frequency modulation circuit, and more particularly to a pulse width modulation inverter circuit and a method thereof to modulate a pulse width according to a voltage of input direct current power source.
2. Description of the Related Art
Cold cathode fluorescent tubes are a lower-press mercury vapor discharge tube. When voltages are applied to two terminals of a cold cathode fluorescent tube, mercury vapor charges and gas atoms would run into each other to generate ultra-violent (UV) light. The UV light excites fluorescent material on the wall of the tube to emit visible light. Since a filament is not used, the burn-down or break-down of the filament can be avoided. Therefore, the cold cathode fluorescent tube has substantial life span.
In a stable operation, the cold cathode fluorescent tube requires a power frequency from about 30 KHz to 80 KHz and the power has a sinusoidal wave without direct current component. The operational voltage of the tube is almost a constant. The brightness of the tube is determined by the tube current flowing through. The voltage to turn on the tube is 2 to 2.5 times of the stable operational voltage. The turn-on voltage and the operational voltage of the cold cathode fluorescent tube depend on the size of the tube. For 14-inch or 15-inch LCDs, the turn-on voltage of the cold cathode fluorescent tube is about 1400 Vrms. When the maximum rated current of the tube is about 7 mA, the operational voltage is about 650 Vrms.
Generally, a typical cold cathode fluorescent tube uses the inverter in the pulse frequency modulation circuit to invert the DC-input voltage to the AC-output voltage to drive the cold cathode fluorescent tube. In order to stabilize the operational current of the cold cathode fluorescent tube, a resonance tank is used in the pulse frequency modulation circuit to properly adjust the output current of the pulse frequency modulation circuit.
FIG. 1 is configuration showing a relationship between operational frequencies and corresponding output voltages in a resonance tank. Referring to FIG. 1, with different operational frequencies, the output voltages of the resonance tank are different. When the input frequency is equal to the resonant frequency of the resonance tank, the output voltage of the resonance tank reaches the maximum value Vmax. For example, when the operational frequency of the cold cathode fluorescent tube is F1, the output voltage is V02. When the operational frequency of the resonance tank is reduced and close to the resonant frequency F2, the output voltage is increased to V01. By using the relationship between the output voltages and input frequencies, the pulse frequency modulation circuit modifies the operational current of the cold cathode fluorescent tube by changing the operational frequency.
FIG. 2 is a circuit block diagram showing a conventional pulse frequency modulation inverter circuit. Referring to FIG. 2, the power switch 203 is coupled to the DC power source 201 and to the resonance tank 205. The input voltage is then applied to the transformer 207 to drive the cold cathode fluorescent tube 209 to illuminate.
Referring to FIGS. 1 and 2, when the operational frequency of the resonance tank 205 is F1, the operational current to drive the cold cathode fluorescent tube 209 is small. Then, the fluorescent tube current detector 211 outputs a detecting signal to the negative feedback controller 213. The negative feedback controller 213 outputs a feedback voltage signal to reduce the frequency of the voltage control oscillator 215 so that the oscillation signal frequency outputted from the voltage control oscillator 215 is reduced.
The fixed pulse width power switch driver 217 outputs the fixed pulse width signal, which varies with the oscillation frequency outputted from the voltage control oscillator 215. When the oscillation signal frequency is reduced, the frequency of the fixed pulse signal is also reduced. The operational frequency inputted to the resonance tank 205 is near the resonant frequency of the resonance tank 205 to achieve the purpose of increasing the output voltage of the resonant voltage. In contrary, if the current of the cold cathode fluorescent tube 209 at the operation of frequency F1 becomes larger, the system adjusts the operational frequency inputted to the resonance tank 205 and makes the operational frequency shift away from the resonant frequency of the resonance tank 205 and higher than the operational frequency F1. Accordingly, the large current can be reduced.
The change of the operational frequency of the resonance tank 205 may effectively adjust the operational current of the cold cathode fluorescent tube. However, from FIG. 1, the output voltage of the system operating near the range of the resonant frequency of the resonance tank 205, are increased, and so is the efficiency. But, the output voltage and the efficiency of the system operating far from the range of the resonant frequency of the resonance tank 205 are decreased. Accordingly, in the system with DC power sources with a larger voltage range, when the pulse frequency modulation operates under a high input voltage, the frequency needs to be far away from the resonant frequency to stabilize the current outputted to the cold cathode fluorescent tube. In this method, high input voltage is applied to the circuit, but the output efficiency of the circuit is low, which is not cost effective.